In experimental cultures a good infection is obtained by inoculating the seed with a suspension of spores. The seed is washed and dried to loosen the hulls around the kernel, the suspension is mixed with the grain, and the moist inoculated seed is stored for 16 to 20 hours to promote spore germination and the spread of mycelium under the hull.

Higher infection has been obtained in plants kept under mild temperatures for 2 to 4 weeks after the seedlings emerged than when the plants were grown continuously under cool conditions like those of fall and winter. During the initial infection, between seed germination and the emergence of the seedling, the highest percentages of smutted plants are obtained at soil temperatures of 50 to 70 F.

The amounts of smut tend to be higher in plants grown in acid soil than in neutral or alkaline soil. Soil type, soil compaction, depth of seeding, and the rate of seedling growth also may affect development of smut.

Thirteen physiologic races of the barley covered smut fungus have been isolated by their reactions on eight varieties of spring barley. One prevails in the winter barley region, another in the Mississippi Valley, and a third race along the Pacific coast. Several varieties have shown high resistance to covered smut. Two varieties, Ogalitsu and Jet, are resistant to 13 races of covered smut, 9 races of nigra loose smut, and 6 races of nuda loose smut. Covered smut may be controlled by treating the seed with formaldehyde or organic mercury dusts.

NIGRA LOOSE SMUT of barley, Ustilago nigra, was first discovered in 1932 by V. F. Tapke, who separated it from ordinary loose smut. The two loose smuts, which often were mixed together in a field, previously had been regarded as a single type that infected barley only through the flowers. The two loose smuts look alike, but the nigra pathogen is a seedling-infecting smut and is controlled by treating the seed with surface disinfectants. When the two loose smuts occur together only partial control can be obtained by surface disinfection because the deep-borne loose smut is not affected by such treatment. The fungi causing barley covered smut and nigra loose smut have been hybridized artificially. Nigra loose smut causes an average annual loss of 1 million bushels of barley in the United States.

The disease usually first becomes noticeable at heading time, when dusty, smutted heads appear. Each contains millions of loosely held, dark-brown microscopic spores. Wind carries the spores. Spores that come in contact with the flowers and young developing seeds of healthy heads then behave like the spores of covered smut. They may lie dormant or germinate and form a sub-hull mycelium. Moisture, temperature, and other conditions affect germination of spores.

Temperatures of 60 to 70 and a relatively dry soil are most favorable to infection of the seedling during the period of emergence.

Immediately after the seedlings have emerged, temperatures of 60 to 70 for 10 to 30 days give higher percentages of smut than do constant low temperatures.

Nine distinct pathogenic strains or physiologic races of the nigra loose smut fungus from this country have been isolated by their reaction on eight varieties of spring barley. Four are rather widespread in the United States. At least four races that do not occur in the United States have been found in smutted barley from various parts of Israel.

At least four varieties of barley are highly resistant to all races of the pathogen thus far found in the United States. None is grown commercially, but all are useful in breeding productive resistant varieties.

ONLY TWO of the small grain smuts are floral-infecting. The smutted heads appear at heading time. They shed their spores during the bloom period of the normal heads. The loosely held spores are distributed by wind, rain, insects, or other agents. Initial infection rarely occurs after the fertilized ovary has attained one-third of its mature size. Spores that come in contact with healthy flowers germinate by forming infection threads, which grow down the pistil or through the ovary wall into the young, developing seeds. A deep infection of the seed follows, so that surface seed disinfectants do little good. The fungi also are unable to produce infection when spores are applied to the surface of ripe seed.

The two floral-infecting loose smuts, one of wheat and one of barley, are practically identical except that one attacks wheat but not barley and the other attacks barley but not wheat. Both are widely distributed in humid and subhumid areas but are less common in dry areas. Infection threads develop too slowly in dry air to penetrate the ovary during its brief period of susceptibility.

Wheat loose smut caused annual losses ranging from 3 million to 18 million bushels in the United States from 1917 to 1939. Loose smut in barley caused annual losses of 750,000 to 4.5 million bushels in those years.

High humidity favors infection by loose smut in wheat and barley, but the ranges of temperature and humidity most favorable to infection have not been determined exactly. Certain wheats that rarely develop loose smut when grown under the dry conditions of western United States are extremely susceptible when grown in humid climates.

The experiments in the Netherlands show that wheat loose smut spores may be disseminated and cause infection up to at least 100 yards from their point of origin. Infection diminished regularly with distance from the source of inoculum where high winds prevailed during the flowering season; it was spotty when air currents were irregular.

A warm soil when seedlings emerge seems more conducive to smut than does a cold soil. In Japan, for example, early seedings of winter wheats and barleys often show higher amounts of loose smut than do later seedings. Grains that follow rice show little smut because seeding then is delayed for at least 6 weeks.

Soil fertility seems to have little influence on infection. Wheat plants grown from infected seed are more susceptible to winter-killing, especially under severe conditions, than are similar healthy plants.

The deep-infecting loose smuts of wheat and barley can be controlled by hot-water seed treatment. The usual method involves immersing the seed 4 to 12 hours in unheated water, followed by a 10-minute dip in water at 129 F. A single immersion in water held at 120 for 95 minutes also is effective in control. Such treatments are so laborious that individual growers rarely use them. The treated seed is soft, swollen, and hard to dry before seeding. The treatment often injures germination. Occasionally central treating plants, established where steam or hot water are available, treat the seed for local growers. Control also has been obtained by soaking the seed for 6 hours in water at room temperature followed by a soak of 40 hours in a 0.2-percent solution of Spergon.

C. S. HOLTON holds degrees from Louisiana State University and the University of Minnesota. He is in charge of the wheat smut program in the Northwest.

V. F. TAPKE has conducted research on cereal diseases since joining the Bureau of Plant Industry, Soils, and Agricultural Engineering in 1918.